1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, a facsimile and a printer which are provided with a heating apparatus, and particularly relates to a heating and fixing apparatus which heats and fixes an unfixed toner image that has been transferred onto a transfer material.
2. Description of the Related Art
A fixing apparatus in an image forming apparatus such as a copying machine, a facsimile and a printer which uses an electrophotographic process is an apparatus for heating and fixing on the transfer material an unfixed toner image, which has been transferred onto a transfer material, by a heating roller, a ceramic heater or the like. Fixing apparatuses that are generally used include a heating-roller type of fixing apparatus, in which a halogen heater is used as a heat source, and a film heating type of fixing apparatus, in which a ceramic heater is used as a heat source. In these fixing apparatuses, a heater is connected to an AC power supply through a switching control element such as a triac, and the AC power supply supplies electric power to the heater. The fixing apparatus is provided with a temperature detecting element, for instance, a thermistor temperature-sensitive element. An engine controller controls the switching control element by turning a switch ON or OFF based on information on the temperature that has been detected by this temperature detecting element. As a result, the engine controller supplies or shuts off the electric power to the heater, and controls the temperature of the fixing apparatus so that the temperature reaches a target fixed temperature (for instance, see Japanese Patent Application Laid-Open No. 2007-052176).
However, in the fixing apparatus, the power consumption of the heater is large, and accordingly, the voltage of a commercial power supply, which is in a primary side of the apparatus, is not converted into a voltage for a secondary side of the apparatus, but is directly used as-is for driving the heater. On the other hand, a control circuit for driving the heater of the fixing apparatus, for instance, such as a temperature detecting element and an engine controller which controls the heater by turning the heater on and off, based on the information on the temperature that has been detected by this temperature detecting element, functions as a circuit in the secondary side, which a user can directly touch. The control circuit in the secondary side is connected to a personal computer (PC) or network equipment. Accordingly, the distance between the heater and a heater driving circuit in the primary side, and the temperature detecting element and the control circuit in the secondary side, needs to be sufficiently secured, in order to protect the user. In addition, an expensive insulating material and an expensive insulating component need to be provided in between the element in the primary side and the element in the secondary side, and as a result, the apparatus tends to increase in price and increase in size.
In addition, as shown in Japanese Patent Application Laid-Open No. 2007-052176, when the temperature detecting element is provided in the primary side, the information on the temperature needs to be transmitted from the temperature detecting element to the engine controller and the like, which exist in the secondary side. For this reason, when the temperature detecting element is provided in the primary side, there is a possibility that the circuit configuration becomes complicated, because a PWM control which uses another component such as a comparator may be needed, for instance.